366 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tol.Yl. 



are without enamel inflections, and the triturating surface exhibits two 

 external and one internal crescentic sections of the investing enamel. 

 On the second superior molar there are three external crescents, and 

 the first molar is simply conic. Between the inner and external cres- 

 cents there are the curved edges of enamel plates directed obliquely and 

 transversely. The grinding surfaces of the inferior molars display, in 

 the unworn condition, curved transverse crests, connected longitu- 

 dinally on the median line ; on wearing, the lateral emarginations of 

 the enamel become shallower, disappearing from the inner side, but 

 remaining on the outer. Incisor teeth not grooved. Foramen infra- 

 orhitale anterius small, inferior, and near the orbit. Postorbital pro- 

 cesses ; no sagittal crest. 



The characters of the dentition of this genus resembles those of the 

 genus Pteromys, which is now confined to Asia and the Malaysian 

 Archipelago. The superior molars differ from those of Fteromys in 

 wanting all re- entrant enamel inflection. 



The general characters of the skeleton are unknown. A femur is 

 rather slender, and a tibia rather elongate, showing that the limbs are 

 not short. 



Four species of this genus are known to me, all from the Truckee 

 Miocene of Oregon. They differ considerably in the details of the struct- 

 ure of the molar teeth. Their more prominent characters may be set 

 forth as follows : 



I. Superior molars short-rooted, witli the external face plane ; inferior molars with a 



prominent median transverse crest. 

 Smaller ; dental crests fewer, simple, not crenate M. Mppodus. 



II. Superior molars long-rooted ; external face of crowns inflected forming two Vs ; 



no median transverse crest on inferior molars.* 



a. Crests of superior molars fewer, simple, not crenate. 

 Larger; crowns short M. liolophus. 



aa. Crests of superior molars more numerous and much crenate. 



Smaller ; plications of inferior molars shallow ; borders raised M. cavatus. 



Larger ; plications of inferior molars profound M. nitens. 



There is a suggestive resemblance between the forms of the molar 

 teeth of the Meniscomys Mppodus and those of the Haplodontia rufa now 

 living in Oregon. The two genera have doubtless had a common origin, 

 but the present differences are considerable. Thus the Haplodontia has 

 an extended osseous cavum tympani, which does not exist in Meniscomys. 



This genus appears to be referable to the Sciuridce. 



ISCHYROMYS Leidy. 



Proceed. Acad. Nat. Sci. Philad. 1856^ p. 89 ; Extinct Fauna Dakota and Nebraska 

 335. — Coloiaxls Cope, Paleontological Bulletin, No. 15, p. 1. 



Char. gen. — The essential features are, dentition, I., i; C, %; M., I5 

 the molars with two crescents on the inner side above, each of which 



*Not seen in M. Uolophus. 



